Defrosting controller for electric refrigerator



Dec. 22, E970 ZENJI KUSUDA ETAL 3,548,609

DEFROSTING CONTROLLER FOR ELECTRIC REFRIGERATOR Filed Dec. 9, 1968INVENTOR s 2 Em; w 54 09 ATTORNEYS United States Patent 3,548,609DEFROSTING CONTROLLER FOR ELECTRIC REFRIGERATOR Zenji Kusuda,Ibaragi-shi, and Takeji Kobayashi, Kyoto,

Japan, assignors to Matsushita Electronics Corporation, Osaka, Japan, acorporation of Japan Filed Dec. 9, 1968, Ser. No. 782,268 Claimspriority, appljgation Japan, Dec. 14, 1967,

US. Cl. 62-156 Claims ABSTRACT OF THE DISCLOSURE This invention relatesto a defrosting controller for electric refrigerators which controlselectronically the defrosting operation of electric refrigerators anddetects a variation in the temperature within the freezer during thedefrosting step, thereby automatically bringing the defrosting operationto an end.

It is commonly known that the cooling efiiciency of an electricrefrigerator is reduced when frost accumulates within the freezer, ormore specifically, when frost deposits on the surface of the evaporatorin the electric refrigerator. Measures previously taken for the removalof frost are such that a defrosting valve is opened for a predeterminedperiod of time for circulating hot gas through the evaporator or aheater is placed in operation to generate heat for a predeterminedperiod of time. However, the prior defrosting methods in which theduration of the defrosting operation is thus predetermined and thedefrosting operation is continued for a predetermined period of timeindependently of the amount of frost accumulated within the freezer havehad certain inherent inconveniences. More precisely, even when frost ina small amount is accumulated within the freezer, the defrostingoperation is still continued after the frost has completely been removedresulting in a rise of the temperature in the provisional compartment toa point which is undesirably high, while when quite a large amount offrost is accumulated within the freezer, the defrosting operation isended before all the frost within the freezer can completely be removed.

It is therefore an object of the present invention to overcome theinconveniences encountered with these and other known defrosting systemsand to provide a novel defrosting controller which acts to automaticallystop the heating operation of the defrosting heater as soon as frostaccumulated within the freezer is completely removed.

Another object of the present invention is to provide a defrostingcontroller which brings the defrosting operation to an end by closing ahot gas circulating valve.

The above and other objects, features and advantages of the presentinvention will be apparent from the follow ing detailed description of afew preferred embodiments thereof taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is an electrical circuit diagram of an embodi- Patented Dec. 22,1970 ment of the defrosting controller according to the presentinvention; and

FIG. 2 is an electrical circuit diagram of another embodiment of thepresent invention.

In accordance with the present invention, a thyristor, such as a siliconcontrolled rectifier is connected in series with a heating meansdisposed in a main circuit as shown in FIG. 1, and the siliconcontrolled rectifier is urged to its conductive state or cutoff statefor closing or opening the main circuit thereby placing the heatingmeans in operation when so required.

Referring to FIG. 1, a heating means 3 such as a heater is connectedwith AC. power source terminals 1 and 2. The means 3 may be a relay fordriving a hot gas valve. The defrosting controller comprises a siliconcontrolled rectifier 4, a transistor 5 which is turned ON and OFF whenthe silicon controlled rectifier 4 is OFF and ON, respectively, a DC.bias supplying means 6 for the transistor 5, a thermistor 7 thermallycoupled to the interior of the freezer for detecting a variation in thetemperature within the freezer, a base bias dividing resistor 8, voltagedividing resistors 9 and 10 for dividing the AC. voltage when thesilicon controlled rectifier 4 is in its cutoff state, a diode 11 forrectifying the voltage divided by the resistors 9 and 10, a charge-upcapacitor 12, a switch 13 which is normally opened and is urged closemomentarily in starting the defrosting operation, a base resistor 14 forthe transistor 5, and a collector resistor 15 for the transistor 5.

In the circuit described above, the silicon controlled rectifier 4 is inits cutoff state and the switch 13 is in its open position in the normalstate, that is, in the state in which no defrosting is required. Thus,the circuit including the heater 3 therein is in its open state and noheat is generated by the heater 3. In the above state, the AC. powersource voltage appears across the silicon controlled rectifier 4, andthe capacitor 12 is charged by the resistors 9, 10 and the diode 11. Thevoltage appearing across the capacitor 12 is dependent upon the voltagedividing ratio of the resistors 9 and 10. The emitter-base junction ofthe transistor 5 is forward biased by the above voltage so that thetransistor 5 is kept in its conductive state. Accordingly, no gatetrigger current flows to the gate of the silicon controlled rectifier 4which is connected with the collector of the transistor 5, and thesilicon controlled rectifier 4 is still kept in its cutoff state.

The switch 13 may be closed to give the defrosting starting instructionto the defrosting control circuit which is kept in the state describedabove, thereby placing the defrosting control circuit in defrostingoperation. The purpose can easily be accomplished by associating a meanssuch as a timer with the switch 13 so that the switch 13 canperiodically be closed by the action of the timer.

Suppose now that the switch 13 is momentarily closed by the action ofthe timer. Then, the electric charge accumulated in the capacitor 12 isdischarged through the closed switch 13 to cut off the transistor 5. Asa result, a gate trigger current flows to the gate of the siliconcontrolled rectifier 4 to urge the same into its conductive state. Thisresults in the closure of the main circuit comprising the AC. powersource terminal 1, heater 3, silicon controlled rectifier 4, and AC.power source terminal 2. The heater 3 is thus energized to start removalof frost accumulated within the freezer.

Since the silicon controlled rectifier 4 is urged to its con ductivestate in a moment and the forward voltage drop across the conductingsilicon controlled rectifier 4 is quite small or in the order of severalvolts, voltage of such a magnitude as will urge the transistor 5 to itsconductive switch 13 is forced open again. Thus, the transistor 5 is 3continuously kept in its cutoff state and the defrosting control circuitis held in the operating state which is entirely the same as when theswitch 13 is kept continuously closed. Therefore, the heater 3 continuesto generate heat and the defrosting operation for the removal of frostwithin the freezer is continued.

As the frost accumulated within the freezer is removed, the temperaturewithin the freezer rises gradually and the resistance of the thermistor7 thermally coupled to the interior of the freezer decreases gradually.As a result, the base-emitter junction of the transistor 5 is deeplyforward biased by the voltage obtained by dividing the voltage of theD.C. bias supplying means 6 by the thermistor 7 and the resistor 8. At apoint at which the temperature within the freezer reaches the defrostingceasing temperature, the transistor 5 makes a transition from its cutoffstate to its active region, resulting in a decrease of the gate triggercurrent supplied to the silicon controlled rectifier 4.

Therefore, the conduction angle of the silicon controlled rectifier 4 isnow less than 180 and there appears a phase in which no conduction takesplace. The capacitor 12 is charged during the non-conduction phase ofthe silicon controlled rectifier 4, and a voltage appears across thecapacitor 12. This voltage is also applied to the base of the transistor5 so that the transistor 5 approaches its conductive state more andmore. In the meantime, the socalled positive feedback occurs in thesilicon controlled rectifier 4, in which the gate trigger current isfurther reduced and the conduction angle of the silicon controlledrectifier 4 becomes smaller. As a result, the transistor 5 is rapidlyurged to its conductive state and the silicon controlled rectifier 4 israpidly urged to the cutoff state to open the main circuit again and tostop the defrosting operation, that is, the heating operation by theheater 3.

After the transistor 5 is urged to the conductive state and the siliconcontrolled rectifier 4 is urged to the cutoff state, the temperaturewithin the freezer is reduced and the transistor 5 is deeply biased bythe voltage charged in the capacitor 12 in spite of any increase in theresistance of the thermistor 7 and is not urged to its cutoff stateuntil the switch 13 is closed again. Thus, the circuit is kept in thestate of cooling operation.

While the above description has referred to a defrosting control circuitemploying a thyristor in the form of a P- gate silicon controlledrectifier, it will be understood that a defrosting control circuitsimilar to the one above may be obtained by employing an N-gate siliconcontrolled rectifier.

FIG. 2 shows a defrosting control circuit employing such an N-gatesilicon controlled rectifier. The N-gate silicon controlled rectifier isindicated by the reference numeral 16. As is apparent from FIG. 2, atransistor 17 has a polarity opposite to that of the transistor 5 shownin FIG. 1. More precisely, the transistor 17 is of the pnp type and isconnected with a D.C. bias supplying means 6, a diode 11 and a capacitor12 in a relation entirely opposite to the relation shown in FIG. 1. InFIG. 2, like reference numerals are used to denote like parts appearingin FIG. 1. The operation of the transistor 17 and the silicon controlledrectifier 16 in the circuit of FIG. 2 is the same as the operation ofthe circuit of FIG. 1, that is, in the cooling operation, the transistor17 is kept in its conductive state and the silicon controlled rectifier16 is kept in its cutoff state.

Therefore, the silicon controlled rectifier 16 is not triggered tobecome conductive in such a state of the circuit. Then, when a switch 13is the circuit is closed momentarily, the electric charge accumulated inthe capacitor 12 is discharged through the switch 13 to cut off thetransistor 17. As a result, the silicon controlled rectifier 16 is urgedto conduct so that the defrosting operation by a heater 3 is started.Thereafter, an operation entirely the same as that of FIG. 1 is effectedto remove frost accumulated within the freezer and the defrostingoperation is automatically stopped as soon as the frost has completelybeen removed.

A bidirectional triode thyristor may be used in combination with thesilicon controlled rectifier in the circuit shown in FIGS. 1 and 2.While the arrangement in FIGS. 1 and 2 is such that the voltageappearing across the thyristor is divided by the voltage dividingresistors and this divided voltage is applied to the base of thetransistor through the charging means consisting of the diode and thecapacitor, it will be understood that an alternative arrangement may bemade in such a way that a voltage appearing across the charging means isfirst divided by the voltage dividing resistors and this divided voltageis then applied to the base of the transistor.

It will be appreciated from the foregoing description that thedefrosting controller according to the present invention is adapted toperform the defrosting operation solely under a predetermined condition.More precisely, the defrosting operation is ended as soon as frostaccumulated within the freezer is completely removed. Thus, thedefrosting operation is always correctly performed irrespective of therelative amount of frost accumulated within the freezer. The abovemanner of defrosting operation completely eliminates the prior problemof an unnecessary and objectionable rise in the temperature of theprovisional compartment or imperfect removal of frost which has beeninevitable with the conventional method of defrosting.

What is claimed is:

1. A defrosting controller for an electric refrigerator having adefrosting heater, a thyristor for controlling the operation of saiddefrosting heater, and control means for triggering said thyristor, saidcontrol means comprising voltage conversion means by which an A.C.voltage appearing across the anode and cathode of said thyristor duringits cutoff state is converted into a D.C. voltage, a transistor to thebase of which the D.C. voltage obtained by said voltage conversion meansis applied, said transistor being urged to its conductive state and toits cutoff state when said thyristor is in its cutoff state andconductive state, respectively, and D.C. bias supplying means forsupplying a D.C. bias voltage to said transistor, wherein a temperaturedetector such as a thermistor thermally coupled to the freezer isconnected with the base of said transistor as a base bias dividingresistor, a switch adapted to be closed in starting the defrostingoperation is connected between the base and the emitter of saidtransistor, and the collector of said transistor is electricallyconnected with the gate of said thyristor.

2. A defrosting controller according to claim 1, in which said voltageconversion means is a diode.

3. A defrosting controller according to claim 1, in which saidtransistor is of the pnp type.

4. A defrosting controller according to claim 1, in which saidtransistor is of the npn type.

5. A defrosting controller according to claim 1, in which said switch isof the momentary-contact type.

References Cited UNITED STATES PATENTS 3,203,195 8/1965 Armentrout62-156 3,222,882 12/1965 Sutton 62-156 3,248,892 4/1966 Sutton 621563,363,429 l/1968 Wechsler 62-156 3,460,352 8/1969 Lorenz 62-155 MEYERPERLIN, Primary Examiner U.S. Cl. X.R. 62--276, 278

